The present invention relates to liquids or solids (hereinafter referred to as material) level detectors and particularly to liquids or solids level detectors that utilize the backscattering of neutrons to detect the level of hydrogen containing liquids or solids in a closed vessel. In the above referenced co-pending application there is described and claimed a portable liquid level detector which utilizes a source of fast neutrons and a detector for slow neutrons. As the detector is moved along the wall of a vessel containing a liquid including hydrogen as one of its components, the level of the liquid is detected by the change in the number of slow neutrons being counted. The number of backscattered slow neutrons is directly related to the number of hydrogen atoms present for slowing the fast neutrons from the source. Thus, as the detector moves from the liquid or solid to the gas phase the number of backscattered slow or thermal neutrons produced will decrease. This decrease in the number of slow neutrons is used to indicate the level of the liquid within the closed vessel.
While the detector described in the co-pending application is useful as a portable instrument for making routine surveys of closed vessels such as those used in a refinery, there is a need for stationary material level detectors. While this need has been filled in the past by float-type level detectors, these are not entirely satisfactory in all processes in a refinery and chemical plants. This is especially true where the material has a relatively high viscosity where the float does not readily respond to the material level. Also, the use of floats requires mechanical connections to the interior of closed vessels that must be installed when the equipment is originally built. The presence of mechanical connections also results in maintenance problems since it is difficult to have access to the location of the float without shutting down the process.
In the past, attempts have been made to use gamma type sources and suitable detectors for detecting levels. The use of gamma sources requires that the source be placed on on side of the vessel while the detector is placed on the other side. Thus, again it is necessary to design the particular vessel with suitable mounting means for the source and detector. Unless relatively large gamma sources are used, it is impossible for the gamma rays to penetrate the thick steel walls of most vessels and thus it is necessary to mount the source inside the vessel so that the gamma rays can travel across the material contained in the vessel. Further, in the case of vessels having baffles or stirring mechanisms or other equipment mounted inside the vessel, it is difficult to install a gamma source so that it will have a straight line or path to the detector. If the gamma ray strikes the stirrer or baffles within the vessel, it is deflected and will not impinge upon the detector.